The field of the invention is catalyst devices each of which comprise a multichannel or honeycomb, porous walled, substrate containing a high surface area oxide washcoat as the support for metal catalyst dispersed on and bonded to the washcoat. Such devices with noble metal catalyst are useful for catalytically converting pollutants in the exhaust gas emitted by an internal combustion engine.
In such field it is commonly and commercially known to provide a washcoat layer on the wall surfaces of the porous walled substrate. The surface area of the washcoat is desirably greater than 50 m.sup.2 /g (or more likely 100 m.sup.2 /g) and preferably at least 150 or 200 m.sup.2 /g. Such substrate is usually a porous ceramic material, such as cordierite, but it can also be a porous metal material (in contrast to nonporous metal foil). These porous materials are customarily of relatively low surface area, e.g. less than 10 or 5 m.sup.2 /g (and, for some ceramic materials, less than 2 m.sup.2 /g), and formed by sintering plastically shaped or formed particulate materials that yield the porous ceramic or metal material of the substrate. Usually the substrate is formed by extrusion and sintering of a plasticized mixture, e.g. into a thin walled honeycomb as described in U.S. Pat. Nos. 3,790,654 and 3,824,196. However, the multichannel substrate can be formed in any other useful configuration and by any other suitable process, e.g. as described in U.S. Pat. Nos. 3,112,184. The washcoat is typically applied to the substrate by dipping the substrate in a slurry, usually in water, of oxide particles that will form the washcoat. Such slurry can also include a dissolved catalyst precursor compound, from which the precursor will adsorb and disperse on the particles, and that will decompose and yield the metal catalyst upon calcining or heat treating the washcoat to bind the latter to the substrate. Such heating also causes the metal catalyst to be dispersed and bonded on the washcoat.
Internal combustion engine performance, e.g. in an automobile, is related to the back pressure effect of the catalytic converter in the exhaust gas conduit extending from the combustion chambers of the engine. Such performance generally improves as the back pressure is decreased.
In order to decrease the back pressure and increase engine performance, the open frontal area (OFA) of the support, i.e. the aggregate open transverse cross sectional area of the flow-through channels or cells of the washcoated, multichannel or honeycomb substrate, should be increased. However, this approach until now has been hindered by problems and accommodated by less than the desired solution.
For example, such approach is limited on the one hand by the necessity of applying an adequate amount (i.e. not too thin layer) of washcoat for sufficient catalytic function of the device, and on the other hand by maintaining enough wall thickness for structural integrity of the support. A relatively great amount of high surface area oxide is necessary in order to accommodate a sufficient amount of noble metal catalyst dispersed and bonded thereon, which would not be provided by a decreased quantity of high surface area oxide layer.
A known approach to avoiding the washcoat layer taking up space in the channels or cells of the support is the manufacture of the substrate with the washcoat material mixed with the particulate material for the structural (e.g. ceramic) material so that the formed and sintered product is the catalyst support with the washcoat particles embedded in and distributed through the walls as described in U.S. Pat. Nos. 4,637,995 and 4,657,880. A catalyst may subsequently be deposited on those washcoat particles. Additionally, such washcoat particles may have metal catalyst deposited on them before being mixed with particles of the structural material and embedded in the walls as described in U.S. Pat. No. 4,888,317. In either form, the resulting catalyst device can be characterized as a catalyst-in-wall structure. To date, such a catalytic converter device has been found not to provide catalytic activities as good as catalytic converter devices with the conventional type of washcoat layer on the wall surfaces.
The washcoat materials known or accepted, prior to the new invention described herein, for suitable support of noble metal catalysts yielding desired catalytic activities generally contain substantial amounts of oxide particles having particle diameters greater than 1 .mu.m. As a consequence of such relatively large particles, it has not been possible to cause a substantial amount of the oxide particles of the washcoat to go into the pores of the walls of the flow-through channels of the porous supports so as to leave a thinner surface layer thereof on the walls of the substrate and thereby yield greater OFA. In typical multichannel or honeycomb substrates, the total open porosity, by volume, is in the approximate range of 5-50% (or more likely 5-25% for metal substrates and 30-45% for ceramic), and the average pore size is in the approximate range of 1-50 .mu.m (or more likely 3-10 .mu.m). Such pores are too small to enable adequate amounts of washcoat particles to enter them.
However, in regard to the back pressure problem, commercial efforts have been reasonably successful over the years in successively reducing the thickness of honeycomb substrate walls from about 0.3 mm to about 0.17 mm and recently to some extent to 0.1 mm. Such wall thickness reductions provide correspondingly increased OFA for substantially similar transverse cross sectional cell density in the supports. The latter efforts were made possible with new substrate material of some improved intrinsic strength to approximately offset loss of bulk strength with thickness reduction.
Some ceramic materials, e.g. cordierite and aluminum titanate plus mullite, of substrates are characterized by advantageously having microcracks in their structure. Such microcracks contribute to higher resistance to thermally induced cracking by allowing the thermally expanding material to reduce their widths, which lowers the overall (i.e. average) thermal expansion of the ceramic substrate material, and thereby avoid thermal stresses that otherwise would develop in the material. However, washcoating of such microcracked substrates can cause a serious problem. U.S. Pat. Nos. 4,451,517 and 4,532,228 reveal that washcoats fill the microcracks and obstruct their beneficial function during thermal shock conditions. Such obstruction during repeated heating and cooling of the converter causes strains and cracking induced by thermal expansion of the substrate material that is not allowed to reduce the width of the obstructed microcracks. As the solution to this obstruction problem, these patents teach the filling of the microcracks with organic materials before applying the washcoat on the wall surfaces of the substrate, so that the washcoat cannot enter the microcracks, and then burning out the organic binder while calcining or heat treating the washcoat to bind it to the wall surfaces of the substrate.
These prior art teachings indicate that it should not be usefully possible to deposit a washcoat mainly in the pores of the walls of multichannel or honeycomb substrates and support a metal catalyst on the washcoat to effect catalytic activities at least comparable to the activities of conventionally washcoated converters known prior to the invention described herein.
In washcoating porous walled substrates with the prior known slurries of the high surface area oxide particles, where a substantial portion of those particles have particle diameters larger than 1 .mu.m, typical formulations of the slurries have desirably high solids content of spray dried boehmite or calcined gamma alumina and, consequently, are characterized by relatively high viscosity. In some cases the slurries include dissolved noble metal precursors or compounds. Such slurries behave in a conventional slip casting mode, wherein their composition and flow properties significantly change during the coating operation. Such behavior presents a problem. After dipping a number of the substrates in such slurry, a disproportionate amount of water is taken up by the substrates. The slurry remaining after washcoating a number of substrates is depleted of water such that its viscosity is too high for continued use in washcoating. It is often difficult to reclaim such depleted slurry as it is not always easy to add water to it in a manner to reconstitute the necessary uniform slurry composition with uniform viscosity.